Apparatus and method for in-vivo fat and iron content measurement

ABSTRACT

An apparatus for measuring liver fat content and/or iron content non-invasively in vivo using unilateral NMR technology is disclosed. The apparatus comprises a unilateral NMR probe positioned against or in the proximity of the body. The probe generates static and RF magnetic fields to measure NMR signals at selected depths into the body. The apparatus further comprises a unilateral magnet and a unilateral antenna placed between a magnet and the body or around the magnet. The unilateral antenna is used to attain NMR signals. The measurement of NMR signals at a single position with a single frequency determines fat content on a specific volume inside the liver. The measurement of NMR signals at a single position with two or more frequencies determines iron content on a specific volume inside the liver.

REFERENCE TO PRIOR APPLICATIONS

The present disclosure claims priority from U.S. provisional patent application 62/456,164 filed on Feb. 8, 2017, the disclosure of which is provided therewith.

FIELD OF THE INVENTION

The present invention generally relates to medical devices. More particularly, the present invention relates to a low cost, compact Nuclear Magnetic Resonance diagnostic device for measuring in-vivo and non-invasively the fat and/or iron content in organs in the human body such as liver.

DESCRIPTION OF RELATED ART

Liver fat and iron content may be measured accurately using Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI) methods. With conventional MRI methods, the fat and iron content is measured by looking at selected pixels in the liver. With NMR spectroscopy and MRI spectroscopy (MRIS), the fat signals may be discriminated from signals from surrounding tissues based on a proton spectrum. Protons are abundant in both, fat (triglycerides) and in the surrounding tissues. The relative concentration of fats may be based on multi-component analysis of the NMR signals, with low spectral resolution instruments.

Further, echo train techniques are well known in NMR and are commonly used with low spectral resolution instruments. Iron content is determined in a similar fashion, but doing the NMR measurements using at least two magnetic field strengths. The correlation of iron content to NMR measurements at various fields is well recorded and has been demonstrated using analyses with multiple clinical MRI scanners.

Open or unilateral NMR probes have been developed and used for industrial and food applications, including the measurement of fat in meats.

With the significant increase in metabolic syndrome incidence, it had become critically important for internal medicine and specialist practitioners to be able discriminate patients at highest risk for severe complications, such as type II diabetes and liver cirrhosis. These trends suggest the need for tools for safe, non-invasive, and inexpensive assessment of liver disease.

Non-alcoholic fatty liver disease (NAFLD) is the most common hepatic disorder in the US. A subset of patients with NAFLD develops non-alcoholic steatohepatitis (NASH, abnormal retention of lipids), leading to cirrhosis, and the eventual need for transplant. Further, a large number of patients with normal weight suffer from multiple aspects of metabolic syndrome including NAFLD and NASH. Disease recognition is often delayed in these patients, relative to obese patients, leading to more severe complications.

Assessment of hepatic steatosis for clinical care requires diagnosis and grading of severity. The relevant classification threshold may vary, from the standard 5% steatosis threshold defining hepatic steatosis, to the 30% threshold for exclusion of liver transplantation donors. Accurate quantification is necessary for grading steatosis and for longitudinal monitoring of patients.

Iron overload occurs in liver disease, metabolic syndrome and hereditary hemochromatosis, in hemodialysis patients receiving supplemental iron, and in patients who receive multiple red-cell transfusions for thalassemia, sickle-cell disease (SCD), and myelodysplastic syndrome (MDS). It can cause death from heart failure, liver cancer or cirrhosis, as well as diabetes, endocrine deficiency and joint problems. It may increase risks of hepatocellular cancer in alcoholic liver disease (ALD) and NASH, exacerbate fibrosis in ALD, affect insulin resistance and liver dysfunction in some patients with metabolic syndrome, and contribute to immune dysfunction and heart failure in hemodialysis. Together, these conditions affect millions of patients.

Existing methods to quantify iron overload are ambiguous, invasive, or expensive. Typically, clinicians infer iron status from serum ferritin and transferrin saturation. However, these indicators are inherently ambiguous, because they are affected by liver disease, inflammation, hemolysis and other common conditions. As a result, diagnosing iron overload can be a complicated process that may include genetic tests for hemochromatosis, and integrating multiple clinical signs and serum tests to rule out inflammation, liver disease and other confounding factors. Liver iron measurements by biopsy or MRI are more direct and less ambiguous, but liver biopsy is invasive, while an MRI scan is expensive, and requires separate appointment at a heavily booked radiology center.

SUMMARY OF THE INVENTION

Detection and monitoring of health issues related to iron and fat content in the liver are addressed by an apparatus that is a low cost alternative that generates clinically relevant measurements. The apparatus uses non-spectroscopic NMR-based biomarkers of tissue fat concentration and iron content.

The apparatus is placed directly on or near by the body in the proximity of the liver or other organs. The apparatus allows collecting NMR signals without having to place the patient inside a costly large magnet (clinical MRI). The apparatus allows early diagnosis and staging of patients with fatty liver disease and metabolic syndrome, offering a cost-effective solution for single or periodical monitoring. The apparatus allows clinicians to quickly and easily measure fat and iron content within a person's liver.

In one aspect of the present disclosure, an apparatus for measuring liver fat fraction non-invasively in vivo using unilateral NMR technology is disclosed. The apparatus comprises a probe positioned against or in the proximity to the body. The probe generates a sensitive volume inside the body. The apparatus further comprises a unilateral antenna placed between a magnet and the body or around the magnet. The unilateral antenna is used to attain an NMR signal. The probe generates static and Radio Frequency (RF) magnetic fields used to measure the NMR signal at selected depths into the body. The measurement of NMR signal at a single position with a single frequency determines fat content on a specific volume inside the liver.

In another aspect of the present disclosure, an apparatus for measuring liver iron content using unilateral NMR technology is disclosed. The apparatus comprises a probe positioned against or in the proximity to body. The probe generates a sensitive volume inside the body. The apparatus further comprises a unilateral antenna placed between a magnet and the body or around the magnet. The unilateral antenna is used to attain an NMR signal. The probe generates variable static and RF magnetic fields to measure the NMR signal at selected depths into the body. The measurement of NMR signal at a single position with two or more frequencies determines iron content on a specific volume inside the liver.

The features and advantages described in this summary and in the following detailed description are not all-inclusive, and particularly, many additional features and advantages will be apparent to one of ordinary skill in the relevant art, in view of the drawings, and specification thereof. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and may not have been selected to delineate or circumscribe the inventive subject matter.

BRIEF DESCRIPTION OF DRAWINGS

In the following drawings, like reference numbers are used to refer to like elements. Although the following figures depict various examples of the invention, the invention is not limited to the examples depicted in the figures.

FIG. 1 shows an apparatus for measuring liver fat and iron content, in accordance with one embodiment of the present disclosure;

FIG. 2 illustrates an example of the magnetic field distribution for the unilateral NMR probe;

FIG. 3 shows frequency dependency on depth and RF pulse excitation bandwidth;

FIG. 4 illustrates the probe and the magnetic field distribution;

FIG. 5 illustrates position of the probe, the liver and the sensitive volume;

FIG. 6 illustrates patient standing while the liver fat and/or iron measurement is performed;

FIG. 7 illustrates an embodiment of the apparatus for liver fat and iron measurement; and

FIGS. 8 and 9 illustrate another embodiment of the apparatus for liver fat and iron measurement with the table and patient lying on the table.

DETAILED DESCRIPTION

The following detailed description is intended to provide example implementations to one of ordinary skill in the art, and is not intended to limit the invention to the explicit disclosure, as one or ordinary skill in the art will understand that variations can be substituted that are within the scope of the invention as described.

The present disclosure discloses an apparatus to measure the fat and/or iron content in vivo in human organs, in particular, but not limited to the liver. The apparatus uses compact unilateral NMR technology and custom data acquisition and processing methods to provide information to diagnose and monitor liver diseases.

The apparatus is implemented using clinical MRI methodologies used to measure liver fat and iron in humans, but utilizes a compact, low cost diagnostic instrument with a probe placed against or near by the body, in the proximity of the liver.

The apparatus is a portable single-sided or unilateral NMR instrument that measures rapidly and non-invasively the liver fat and/or iron content.

Various embodiments of the apparatus for measuring liver fat and iron content are explained using FIGS. 1-9.

Referring to FIG. 1, an apparatus for measuring liver fat and iron content is shown, in accordance with one embodiment of the present disclosure. The apparatus comprises a permanent or electro magnet 101, an antenna 102, a probe enclosure 103, a body of the patient 104, a sensitive volume 105, and a liver 106.

The apparatus 100 is placed on or near the patient, in the proximity of the liver, in order to measure liver fat and/or iron content. This approach benefits from quantitative MRI techniques in terms of demonstrated ability to measure fat and iron content in human tissue. It introduces the possibility to perform rapid diagnosis at a fraction of the per-test cost and without the need of dedicated facilities.

The apparatus 100 uses a unilateral NMR probe 103 positioned directly on the body of a patient, as shown in FIG. 1. The unilateral magnet 101 and antenna 102 generates a static magnetic field that may be produced by a controlled-current electromagnet, by a permanent magnet array, or by a combination of these two. The apparatus 100 contains RF transmitter and receiver coils or antennas. These may be separate elements or a combined transmit-receive element. The single-sided permanent magnet and the open RF antenna project safe magnetic fields into the human body, alike whole body clinical MRI, but without having to place the patient inside of a large scanner.

As the NMR measurement is performed in the presence of inhomogeneous fields, the magnetic field inhomogeneity is used to achieve spatial resolution and to measure diffusion coefficients. Diffusion coefficients help discriminates between the various tissues present in a selected volume inside the human body.

The use of various frequencies or magnetic fields to determine iron content has been well established in clinical MRI. The field variations in a selected sensitive volume may be achieved, for example, by varying the field generated by a unilateral electromagnet or by repositioning a permanent magnet.

NMR

For purposes of the present description, the volume within which meaningful results will be produced is referred to as the sensitive volume 105. The magnetic field causes nuclear spins within the sample to effectively line up parallel to the field direction—or to polarize. This orientation is permuted by exciting those nuclei with one or a series of RF pulses. The RF pulses are generated with the antenna 102. As these excited nuclei realign to the external magnetic field, they emit a RF signal that is detected by a receiver coil (antenna). The receiver coil may be the same transmitter antenna 102 or a separate element. The frequency of the signal the nuclei emit is proportional to the strength of the external magnetic field. The signal lifetime depends on the mobility of the nucleus and the composition of its surrounding tissue.

Unilateral NMR

In single-sided or unilateral NMR, the magnet and coil (antenna) have a single-sided configuration, allowing to collect NMR signals from a region removed from the probe 103. The static magnetic field may be generated by a permanent magnet, an electromagnet, a superconducting magnet, or a combination of those. The magnetic field is projected in a region outside the probing head, into the liver 106.

Referring to FIG. 2, the size of the sensitive volume is determined by the bandwidth of the RF pulses and the magnetic field distribution in the presence of inhomogeneous fields. As can be seen in FIG. 2, an example of the magnetic field distribution for the unilateral NMR probe is shown. “y” represents the direction outwards from the magnet, into the body. Depth Range is indicated by numeral 201, Magnetic Field Amplitude is indicated by numeral 202, and Depth of Penetration is indicated by numeral 203. The depth range is determined due to the fact that the excitation frequency is proportional to the static magnetic field, and the RF pulses have a limited frequency bandwidth.

The unilateral probe used in the liver disease diagnostic device is flat or slightly concave, permitting access from one side of the body, in the proximity of the liver. The probe generates a sensitive volume inside the body. The NMR signal is attained with a unilateral antenna positioned between the magnet and the patient (as shown in FIG. 1) or around the magnet. The positioning of the sensitive volume is determined by the distribution of the static and the RF magnetic fields generated by the magnet and transmitter coil and by the frequency.

The extension and position of the sensitive volume may be changed by frequency encoding using a single, broadband excitation pulse or a series of pulses with different frequencies. Referring to FIG. 3, left side shows Excitation of the depth of interest with a single, broadband RF pulse. The frequency (f) dependency with depth (y) is determined by the distribution of the static magnetic field. Right shows Excitation of the depth of interest with three RF pulses of slightly different frequency. The frequency (f) dependency with depth (y) is determined by the distribution of the static magnetic field change in sensitive volume. Numeral 301 indicate frequency Response, Depth of Penetration is indicated by numeral 302, Frequency is indicated by numeral 303, and Pulse Bandwidth is indicated by numeral 304.

The Q of the probe is a measure of the electromagnetic response bandwidth of a resonant circuit. A relatively low quality factor (Q) probe is used for broad bandwidth pulses.

To ensure the sensitive volume is in the liver, it is necessary to reposition the probe, altering the gap between the body and the probe, change the frequency to excite a different region as the field is non-uniform and proportional to the frequency, or change the field strength keeping the frequency unchanged.

Further, in order to measure the iron content, NMR signals are measured at different fields. Therefore the use a controlled-field electromagnet is preferred for the iron content measurement.

The preferred approach used in the invention focuses on a unilateral magnet and probe to achieve depth resolution without repositioning the probe.

The unilateral probe has two main components:

A magnet generating a static magnetic field

An RF coil or antenna generating an RF field mainly perpendicular to the static field.

The magnet may be composed of a permanent magnet array, an electromagnet or a combination of the two. The magnet projects a strong (typically 0.1 to 0.5 Tesla) static magnetic field in a region outside of the flat or concave array.

Error! Reference source not found., a configuration using two tapered permanent magnet blocks with opposite magnet orientations is shown. As can be seen, numeral indicates Magnet Block 401, Magnetic Block Orientation 402, Antenna 403, RF Field Orientation 404, Sensitive Volume 405, and Static Field Contour Lines 406.

The field of such opposite-block configuration is mainly parallel to the outer surface of the probe. The probe configuration may be optimized to produce flat surfaces of constant magnetic field amplitude. This has an advantage in terms of definition of a disc-shaped sensitive volume and helps on the computation of diffusion coefficients.

The RF antenna may be a simple loop or spiral coil placed above or in the gap of the magnet array. The antenna may also be placed around the magnet array. An antenna array may provide a more effective way to transmit and receive signals to and from the sensitive volume. Given the conductivity of the human tissue, the unilateral antenna should preferably have a low Q factor, so detuning in the proximity of the body is not as prominent. Also, the RF antenna may be retuned manually or automatically after the probe is placed on or close to the body, correcting for tuning shifts generated by the presence of the electrical conductivity of the body.

Error! Reference source not found. is a representation of the positioning of the open probe in the proximity of the liver. The sensitive volume of the unilateral probe is in the liver. Specifically, FIG. 5 shows an Open Magnet Array 501, an RF Antenna 502, Sensitive Volume 503, an RF In/Out Line 504, and position of Liver 505.

For the in-vivo unilateral NMR method, the probe is placed on or near by the body, in the proximity of the liver. A scan is performed while keeping the compact probe in place, without the need to apply pressure on the skin. The scan is performed at a single, selected depth or a depth profile is manually or automatically attained (as shown in FIG. 3), by changing the frequency (proportional to the static magnetic field) or repositioning the probe.

The sensitive volume of the probe is outside of a compact unilateral magnet. An unilateral RF antenna is placed between the sensitive volume and the magnet or in the gap of the magnet array. Depth resolution is attained by frequency encoding the acquired signal using a single, broadband excitation pulse or by using a series of pulses with different frequencies. The probe generates static and RF magnetic fields in a manner that NMR measurements are performed at selected depths into the body. The measurement may be performed at a single position and with a single frequency, determining the fat content on a specific volume inside the liver or other organs. Measurements may also be performed at various frequencies in a single position (for example by changing the field strength generated by an electromagnet) to determine the iron content. The use of various frequencies to determine iron content has been well established in clinical MRI.

The time series of the NMR signal when using a sequence of RF pulses—for example Carr-Purcell-Meiboom-Gill (CPMG)—contains NMR relaxation times and diffusion information. The fat content is determined by a combination of NMR amplitude, relaxation times (e.g. T1 and T2) and diffusion coefficient. The use of signal amplitude and relaxation time in a multi-component time series has been well established using clinical MRI devices. The use of diffusion coefficients improves discrimination between the various tissues present in the sensitive volume.

In unilateral NMR, the NMR parameters are measured in the presence of magnetic field gradients. Well known to NMR, various pulse sequences may be used to measure biological, chemical, and physical properties of the tissue. For example density, relaxation times, and diffusion are quantified with so-called spin echo sequences. A common parameter used for the determination of liver fat content with MRI is the Proton Density Fat Fraction (PDFF), which is the ratio of the number of protons of mobile triglycerides and the number of protons of mobile water and mobile triglycerides. Reportedly, this parameter may be determined based on a multi-exponential analysis of the signal amplitude of the NMR signals. The base for the discrimination is the different relaxation times of proton signals from fat, water and tissues. PDFF is a recognized and accurate NMR-based biomarker of tissue fat concentration.

As one of the basic principles of MRI, the NMR signal response is different for the various tissue types. Tissue discrimination is achieved by determining the signal amplitude and characteristics lifetime (or relaxation time) of a multi-component signal. Some relaxation times are biased by the molecular diffusion, which may be used as an additional discrimination factor.

Signal amplitude. The NMR signal amplitude is a measure of the proton density. Protons are present in both the fat and surrounding tissues.

Relaxation times. NMR relaxation times such as the T1, T2 and T2* are a measure of the tissue characteristics at a microscopic level. Effective relaxation times (T2eff) during a sequence of RF pulses and T1 are readily measured with the unilateral instrument.

The invention also uses a parameter that provides additional information on the mobility of the protons in the various tissues: the diffusion coefficient. In the presence of field gradients, the effective decay time during a CPMG pulse sequence is,

1/T2eff=1/T2−A TE ²,

where T2eff is the effective relaxation time (lifetime of the signal on a time series of the inter-pulse signals), T2 is the spin-spin relaxation time, TE is the inter-spin echo duration (echo time), and A is a parameter determined by the magnetic field gradient (G) and the diffusion coefficient (D) of the observed tissue. If the gradient is constant over the sensitive volume, A is proportional to D G². Therefore, the signal decay along a train of RF pulses provides information on T2 as well as diffusion.

NMR signals from protons in fat and surrounding tissues can be characterized and separated by the characteristic of the response, specifically T1, T2 and diffusion coefficients. In this manner, the fat-to-water ratio is computed. NMR measurements at specific depths in the body and depth profiles of NMR parameters quantify the fat content. The measurement of fat content may be performed by using dedicated pulse sequences that enhances or decimates signals with specific relaxation times. As an example, T1 contrast (signal bias) is achieved by changing the repetition rate in the pulse sequence, a well known practice in clinical MRI.

Error! Reference source not found. to Error! Reference source not found. show preferred embodiments of the invention. Error! Reference source not found. and Error! Reference source not found. show a portable configuration where the patient stands while the diagnostic measurement is performed. Referring to FIG. 6, a patient standing while the liver fat measurement is performed is shown. As can be seen, the apparatus comprises a NMR probe head 601, a user interface 602 provided on the apparatus, a height adjustment mechanism 603, a base 604 with case to integrate electronic components, wheels 605, and a patient 606. The wheeled probe 601 is positioned on the body, in the proximity of the liver. The probe 601 height may be adjusted according to the height of the patient. The wheels 605 are locked to avoid displacement of the instrument while the measurements are performed.

Similarly, various views of the apparatus shown in FIG. 6 are presented in FIG. 7. The apparatus comprises a NMR probe head 701, a user interface 702 provided on the apparatus, a height adjustment mechanism 703, a base 704 with case to integrate electronic components, and wheels 705. The probe is tattered to a compact control unit. The apparatus has a control console/user interface, with buttons or a touch screen that triggers a measurement.

An embodiment of the invention uses a high grade permanent magnet array generating magnetic fields parallel to the top of the probe head, with a contained stray field and a moderate depth gradient, helping attain depth profiles. The magnet array may be built with NdFeB magnetic material. Other choices for the material are SmCo and alloys providing temperature coefficients about 30 times lower than that of the broadly used magnetic materials.

The design focuses on safety by keeping the magnetic field confined to a small region around the probe head. Also, when the instrument is not utilized, the probe head may be covered with a ferromagnetic or high permeability material to effectively contain the magnetic field, addressing issues of safety.

Referring to FIGS. 8 and 9, the probe is mounted under a bed—as shown. It generates a sensitive volume above the bed, in accordance with an alternate embodiment of the present disclosure. As can be seen, a patient Table 801, a Probe Opening 802, a Probe and Console 803, Control and Data Line 804, a Retractable Probe Cover 805, and Sensitive Volume 806 are shown. The probe 803 is mounted under the bed and the sensitive volume 806 is located above the bed. The Patient 902 is positioned in a way that the area on the side of the chest —near the liver—is above the Probe and Console 903, as shown in FIG. 9. The patient Table 901 has an integrated Retractable Probe Cover 905 that effectively blocks the magnetic field when is shut—when the apparatus is not in use. This is a safety feature that is easily incorporated using a high permeability or ferromagnetic material that contains the magnetic field when desired.

Based on the above description, the unilateral probe can assess fat content in other organs, particularly skeletal muscle. Since sarcopenia and fatty replacement is a key finding of metabolically obese normal-weight (MONW) subjects, a useful application is tracking fat concentration in limb musculature.

By providing a simultaneous readout of iron concentration and fat fraction, a practitioner could discriminate hyperferritinemia from metabolic syndrome from true iron overload.

In the preceding specification, the present disclosure is described with reference to the specific embodiments. However, it will be apparent to a person with ordinary skill in the art that various modifications and changes can be made, without departing from the scope of the present disclosure. Accordingly, the specification and figures are to be regarded as illustrative examples of the present disclosure, rather than in restrictive sense. All such possible modifications are intended to be included within the scope of present disclosure. 

What is claimed is:
 1. An apparatus for measuring liver fat fraction non-invasively in vivo using unilateral NMR technology, the apparatus comprising: a unilateral NMR probe positioned against or in the proximity to a patient's body, wherein the probe generates a sensitive volume inside the body; whereas the probe consists of an open magnet and a unilateral antenna placed between a magnet and the body or around the magnet, wherein the unilateral probe is used to measure NMR signals at selected depths into the body, and wherein the measurement of NMR signal at a single position with a single frequency determines fat content on a specific volume inside the liver.
 2. The apparatus of claim 1, wherein the fat content in the liver is measured using NMR relaxation times and signal amplitude from multi-pulse sequences.
 3. The apparatus of claim 1, further measures diffusion coefficients of tissue as means to discriminate within various tissues using a unilateral NMR probe.
 4. The apparatus of claim 3, wherein the diffusion coefficient is determined using at least two pulse spacing in a multi-pulse NMR sequence.
 5. The apparatus of claim 1, wherein the probe is positioned against or in the proximity of the body using a unilateral electromagnet.
 6. The apparatus of claim 1, wherein the probe is positioned against or in the proximity of the body using a unilateral permanent magnet.
 7. The apparatus of claim 1, wherein the probe is positioned against or in the proximity of the body using a combination of permanent and electromagnets.
 8. The apparatus of claim 1, wherein the unilateral antenna is mounted under a table to measure fat content.
 9. An apparatus for measuring liver iron content using unilateral NMR technology, the apparatus comprising: a unilateral NMR probe positioned against or in the proximity to a patient's body, wherein the probe generates a sensitive volume inside the body; whereas the probe consists of an open magnet and a unilateral antenna placed between a magnet and the body or around the magnet, wherein the probe generates static and RF magnetic fields to measure the NMR signal at selected depths into the body, and wherein the measurement of NMR signal at a single position with two or more frequencies determines iron content on a specific volume inside the liver.
 10. The apparatus of claim 9, wherein the iron content in the liver is measured using multi-pulse sequences of relaxation times and signal amplitude at two or more frequencies.
 11. The apparatus of claim 9, wherein the probe is positioned against or in the proximity of the body using a unilateral electromagnet.
 12. The apparatus of claim 9, wherein the probe is positioned against or in the proximity of the body using a unilateral permanent magnet.
 13. The apparatus of claim 9, wherein the probe is positioned against or in the proximity of the body using a combination of permanent and electromagnets.
 14. The apparatus of claim 9, wherein the unilateral antenna is mounted under a table to measure iron content.
 15. A method of measuring liver fat fraction using unilateral NMR technology, the method comprises: generating a sensitive volume inside the body; and attaining a NMR signal, wherein static and RF magnetic fields are measures using the NMR signal at selected depths into the body at a single position with a single frequency determines fat content on a specific volume inside the liver.
 16. A method of measuring liver iron content using unilateral NMR technology, the method comprises: generating a sensitive volume inside the body; and attaining a Nuclear Magnetic Resonance (NMR) signal, wherein static and Radio Frequency (RF) magnetic fields are measures using the NMR signal at selected depths into the body at a single position with two or more frequencies determines iron content on a specific volume inside the liver. 